Method and system for management of neighbor scanning

ABSTRACT

Disclosed herein is a method and corresponding system for management of neighbor scanning in a cellular wireless communication system. A radio access network (RAN) sends, and a mobile station receives, a neighbor list update message (NLUM) containing a plurality of different neighbor lists. The mobile station then selects one of the neighbor lists based on a determined first rank-ordering of its active set members. Once the mobile station selects one of the neighbor lists, the mobile station scans the neighbors listed in the selected neighbor list in accordance with the selected neighbor list. If the mobile station detects a change in rank-ordering from the determined first rank-ordering to another rank-ordering, the mobile station may select and transition to another one of the neighbor lists and may begin to scan the neighbors listed in the selected other neighbor list in accordance with the selected other neighbor list.

BACKGROUND

Cellular wireless is an increasingly popular means of personalcommunication in the modern world. People are using cellular wirelesssystems for the exchange of voice and data over such devices as cellulartelephones, personal digital assistants (PDAs), cellular modems, andother mobile stations. In principle, a user can seek information overthe Internet or call anyone over the Public Switched Telephone Network(PSTN) from any place inside the coverage area of the cellular wirelesssystem.

In a typical cellular wireless system, an area is divided geographicallyinto a number of cells provided by a radio access network (RAN). The RANtypically comprises one or more base transceiver stations (BTSs), eachof which has one or more antennas that radiate to define a radiofrequency (RF) radiation pattern. The BTS(s) of the RAN may then becoupled with a base station controller (BSC) or radio network controller(RNC), which may in turn be coupled with a telecommunications switch orgateway, such as a mobile switching center (MSC) or packet data servingnode (PDSN) for instance. The switch or gateway may then provideconnectivity with a transport network, such as the PSTN or the Internet.When a mobile station (such as a cellular telephone, a wirelesslyequipped PDA or personal computer, or another suitably equipped device)is positioned in a cell, the mobile station communicates via an RF airinterface with the BTS of the cell. Consequently, a communication can beestablished between the mobile station and another entity, via the RFair interface and the RAN.

With the explosive growth in demand for wireless communications, thelevel of call traffic in most cells has increased drastically overrecent years. To help manage the call traffic, most cells in a wirelessnetwork are usually further divided geographically into a number ofsectors, each defined respectively by radiation patterns fromdirectional antenna components of the respective BTS, or by respectiveBTS antennae. Herein, cells and sectors are sometimes referred to as“wireless coverage areas.”

In a Code Division Multiple Access (CDMA) wireless system and perhaps inother types of systems, each cell employs one or more carrierfrequencies, and each sector is distinguished from adjacent sectors by apseudo-random number offset (PN offset). Further, each sector mayconcurrently communicate on multiple different channels, distinguishedby “Walsh codes”. When a mobile station operates in a given sector,communications between the mobile station and the BTS of the sector arecarried on a given frequency and are encoded by the sector's PN offsetand, perhaps, a given Walsh code.

According to well known industry standards, a mobile station cancommunicate with a number of “active” sectors at a time. Depending onthe system, the number of active sectors may be up to three or six, forinstance. The mobile station receives largely the same signal from eachof the active sectors and, on a frame-by-frame basis, may select thebest signal to use. A typical mobile station maintains in its datastorage a list of the sectors in its active set (the “active setmembers”). In addition, the mobile station maintains a list of“neighbor” sectors, which are those sectors that are not in the activeset but that are in close vicinity to the mobile station (e.g., thosesectors neighboring the mobile station's active set members). Theseneighbor sectors are collectively referred to as the mobile station's“neighbor set.”

In existing systems, to facilitate a determination of which sectorsshould be in the mobile station's active set, all base stations emit apilot channel signal on each sector, typically at a power level higherthan other downlink signals. A mobile station then regularly measuresthe strength (e.g., E_(c)/I_(o) or signal-to-noise ratio) of each pilotsignal that it receives and notifies the RAN when the strength of apilot signal rises above or falls below respective designatedthresholds. The RAN, in turn, provides the mobile station with anupdated list of active set members.

In one arrangement, for instance, the RAN may transmit to the mobilestation (e.g., over a downlink control channel or traffic channel) aHandoff Direction Message (HDM), containing parameters such as (i) thePN offsets of the mobile station's active set members and (ii) thefollowing handoff parameters that relate to pilot signal strength:

-   -   T_ADD: Threshold pilot signal strength for addition to the        active set (e.g., −14 dB)    -   T_COMP: Difference in signal strength from an active set member        pilot (e.g., 2 dB)    -   T_DROP: Threshold pilot signal strength for removal from the        active set (e.g., −16 dB)    -   T_TDROP: Time for which an active set member pilot falls below        T_DROP to justify removal from the active set (e.g., 2 seconds)

Additionally, the RAN may provide the mobile station with a NeighborList Update Message (NLUM), containing a neighbor list that identifiesthe neighbor sectors of the mobile station's current active set members.In a CDMA system, the neighbor list may identify neighbor sectors atleast in part by PN offset. The mobile station may then monitor all ofthe pilot signals that it receives and may determine if the pilot signalstrength of any neighbor sector exceeds T_ADD by T_COMP. If so, themobile station may send a Pilot Strength Measurement Message (PSMM) tothe base station, indicating the estimated Echo for the neighbor sector,with the neighbor sector identified by PN offset. Depending on currentcapacity and other issues, the RAN may then agree to allow the mobilestation to hand off to the neighbor sector. Accordingly, the RAN mayreserve a channel resource (e.g., a Walsh code) in the neighbor sectorand may send to the mobile station an HDM (i) providing a new active setthat includes the sector having a pilot signal strength that exceedsT_ADD by T_COMP, and (ii) directing the mobile station to use thereserved channel resource in the added active set member. Further, theRAN may send to the mobile station a new NLUM containing a new neighborlist corresponding to the mobile station's revised active set.

After receipt of the HDM that provides the new active set, the mobilestation may send a Handoff Completion Message (HCM) to the RAN,acknowledging the instruction, and providing a list of its active setmembers (identified by respective PN offsets), thereby completing thehandoff.

Similarly, if the mobile station detects that the pilot signal strengthof an active set member drops below T_DROP, the mobile station may starta handoff drop timer. If T_TDROP passes, the mobile station may thensend a PSMM to the RAN, indicating the Echo and drop timer, andsimilarly identifying the active set member by PN offset. The RAN maythen respond by sending to the mobile station an HDM providing a newactive set that does not include the sector having signal strength belowT_DROP. Further, the base station may likewise send to the mobilestation a new NLUM containing a new neighbor list.

In typical practice, the neighbor list that the RAN provides to themobile station will define a priority scanning order of the neighborsectors listed in the neighbor list. The priority scanning order isusually defined in advance (e.g., by network engineers) based on therelative likelihood that the mobile station will hand off to the listedneighbor sectors. In operation, a mobile station will then cyclicallyscan for (i.e., monitor) pilot signals from the various sectors in itsactive and neighbor sets. In one implementation, for example, the mobilestation may (i) scan all of its active sectors and then scan a first(highest priority) sector from its neighbor set, (ii) scan all of itsactive sectors again and then scan a next (next priority) sector fromits neighbor set, and so forth until the mobile station has scanned allof its neighbor set sectors.

OVERVIEW

As noted above, each time a mobile station hands off from one sector toanother, the serving RAN may provide the mobile station with an NLUMcontaining a neighbor list that identifies sectors neighboring themobile station's current active set members and that defines a priorityscanning order of those neighboring sectors (the “neighbors”). Theneighbor list may include any number of neighbors per active set member.For instance, the neighbor list may include an equal number of neighborsper active set member. Alternatively, the neighbor list may include adifferent number of neighbors per active set member based on therelative signal strength of the mobile station's active set members. Forexample, given active set members X, Y, and Z, if the mobile stationdetermines that X has the strongest signal strength, Y has the secondstrongest signal strength, and Z has the weakest signal strength, thenthe neighbor list may include a greater number of X's neighbors than Y'sneighbors, and a greater number of Y's neighbors than Z's neighbors.

Also as noted above, the priority scanning order of a neighbor list maybe based on the relative likelihood that the mobile station will handoff to the neighbors listed in the neighbor list. In one example, forinstance, given the above-mentioned relative signal strength of activeset members X, Y, and Z, the likelihood that the mobile station willhand off to one of X's neighbors may be greater than the likelihood thatthe mobile station will hand off to one of Y's neighbors, which may begreater than the likelihood that the mobile station will hand off to oneof Z's neighbors. Therefore, the priority scanning order may list X'sneighbors first, followed by Y's neighbors, and then Z's neighbors.

After the mobile station receives an NLUM containing a neighbor listthat defines a priority scanning order, and before the mobile stationagain hands off and receives a new NLUM containing a new neighbor listthat defines a new priority scanning, however, the relative signalstrength of the mobile station's active set members may change, perhapsdue to the mobile station changing location. As a result, the receivedneighbor list and priority scanning order may no longer be applicable oroptimal, especially if the neighbor list contains a different number ofneighbors per active set member based on relative signal strength of themobile station's active set members.

Disclosed herein is a method and corresponding system to help deal withthis issue. In particular, according to the method, a RAN will provide amobile station with an NLUM containing a plurality of different neighborlists each optimized for (i.e., corresponding to) different relativesignal strength of the mobile station's active set members. The mobilestation will then select one of the provided neighbor lists, and perhapstransition between the provided neighbor lists, based on the relativesignal strength of the mobile station's active set members. In practice,the mobile station may regularly monitor signal strength of each of themobile station's active set members, and then, upon receiving the NLUMcontaining the plurality of different neighbor lists, the mobile stationmay select a neighbor list based on the current relative signal strengthof the mobile station's active set members. In turn, as relative signalstrength changes, the mobile station may accordingly and dynamicallytransition between the provided neighbor lists.

In an exemplary implementation of the method, a mobile station monitorssignal strength of each of the mobile station's active set members anddetermines a rank-ordering of its active set members based on theirrelative signal strengths. For instance, given active set members X, Y,and Z, if the mobile station determines that X has the strongest signalstrength, Y has the second strongest signal strength, and Z has theweakest signal strength, then the determined rank-ordering of the activeset members is X-Y-Z. Likewise, if the mobile station determines that Xhas the strongest signal strength, Z has the second strongest signalstrength, and Y has the weakest signal strength, then the determinedrank-ordering is X-Z-Y.

Because each of the neighbor lists provided in the NLUM is optimized fordifferent relative signal strength of the mobile station's active setmembers, one neighbor list may be optimized for the rank-ordering X-Y-Zand another neighbor list may be optimized for the rank-ordering X-Z-Y.Thus, if the mobile station determines a rank-ordering of X-Y-Z for itsactive set members, then the mobile station will select the neighborlist optimized for that rank-ordering and scan the neighbors listed inthe neighbor list in accordance with the neighbor list. However, if atsome time after the mobile station selects the neighbor list optimizedfor the rank-ordering X-Y-Z, the mobile station determines a newrank-ordering of X-Z-Y for its active set members, thereby detecting achange in rank-ordering, then the mobile station will transition to theother neighbor list optimized for the rank-ordering X-Z-Y and scan theneighbors listed in that other neighbor list in accordance with thatother neighbor list.

These as well as other aspects and advantages will become apparent tothose of ordinary skill in the art by reading the following detaileddescription, with reference where appropriate to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments are described herein with reference to thefollowing drawings, wherein like numerals denote like entities.

FIG. 1 is a simplified block diagram of an exemplary cellular wirelesscommunication system;

FIG. 2 is a simplified block diagram of various exemplary functionalelements of an exemplary RAN;

FIG. 3 is a simplified block diagram of an exemplary mobile station;

FIG. 4 is a flow chart depicting functions that can be carried out by anexemplary mobile station in accordance with an exemplary method; and

FIG. 5 is a flow chart depicting functions that can be carried out by anexemplary radio access network in accordance with an exemplary method.

DETAILED DESCRIPTION 1. Exemplary Architecture

Referring now to the drawings, FIG. 1 is a simplified block diagram ofan exemplary cellular wireless communication system 100 in which thepresent method may be carried out. It should be understood, however,that this and other arrangements and functions described herein(including in the above overview section) are set forth for purposes ofexample only, and other arrangements and functions can be used insteadand some may be omitted altogether. Further, as in mosttelecommunications applications, those skilled in the art willappreciate that many of the components described herein are functionalentities that may be implemented with hardware, firmware and/or software(e.g., one or more processors executing program instructions), and asdiscrete components or in conjunction with other components, in anysuitable combination and location. Still further, the names of variouscomponents are not intended to indicate association with just one typeof system. Various systems may use analogous components under othernames.

As shown in FIG. 1, the system 100 includes at its core an exemplary RAN102 that is arranged to provide multiple mobile stations with wirelesscommunication service. FIG. 1 depicts one representative mobile station104 by way of example, which could be a cell phone, wirelessly equippedPDA, or any other type of wirelessly-equipped device now known or laterdeveloped. As will be described more below, mobile station 104 isequipped with hardware, software, and/or other logic to communicate withRAN 102 in accordance with an agreed communication protocol, such asCDMA (e.g., IS-95, IS-1040, 1xRTT, 1xEV-DO, etc.), iDEN, WiMAX, LTE,TDMA, AMPS, GSM, GPRS, UMTS, EDGE, or any other communication protocolnow known or later developed.

RAN 102 defines a plurality of sectors (e.g., CDMA sectors operating onrespective PN offsets) in which mobile stations can communicate with theRAN. In particular, FIG. 1 shows RAN 102 radiating to define sevensectors (although the RAN can define any number of sectors), three ofwhich are labeled respectively A, B, and C, and four of which are alllabeled N. In exemplary RAN 102, sectors A, B, and C are active setmembers of mobile station 104 and the four sectors labeled N areneighbor sectors of active set members A, B, and C. RAN 102 may definethese sectors discretely through use of directional antennas as notedabove and/or by various modulation parameters, including, withoutlimitation, carrier frequencies and PN offsets or other parameters,depending on the air interface protocol used. In practice, the sectorsmay overlap to some extent, so that a served mobile station cancommunicate with multiple sectors when at a particular location.

RAN 102 can take various forms and may include any of a variety andnumber of components, depending for instance on the air interfaceprotocol employed by the RAN. For example, the RAN may include one ormore BTSs 106 and one or more BSCs 108. BTSs 106 may include directionalantennas, power amplifiers, and associated transceiver equipmentarranged to establish sectors A, B, C, and N, and to communicate withserved mobile station 104. As such, some or all of the sectors mayemanate from a single BTS (i.e., a single cell site), or various ones ofthe sectors may emanate from separate BTSs.

Each BSC 108 may control one or more BTSs 106 and may manage aspects ofair interface operation. For instance, a BSC may manage assignment ofair interface traffic channels to served mobile stations in response tomobile station origination messages. Further, a BSC may manage handoffof served mobile stations between sectors by engaging in control channelsignaling with the mobile stations. Such control channel signaling mayinclude, for instance, sending and/or receiving HDMs, NLUMs, PSMMs, andHCMs. In one example, BSC 108 may transmit to served mobile station 104(i) an HDM containing parameters such as those noted above, including PNoffsets of the mobile station's active set members and handoffparameters such as T_ADD and T_COMP and (ii) an NLUM that may beincluded in the HDM or may be its own separate message. According to thepresent method, the NLUM will contain a plurality of different neighborlists each optimized for a different respective rank-ordering of themobile station's active set members and each defining a differentrespective priority scanning order of various neighbors. As noted above,at some time after sending the HDM and NLUM, BSC 108 may receive a PSMM(i.e., a trigger for updating neighbor list data in mobile station 104)from mobile station 104 that includes signal strengths detected by themobile station. This may occur, for instance, if mobile station 104determines that a neighbor sector has a signal strength that exceedsT_ADD by T_COMP. In response to receiving the PSMM, BSC 108 may send tomobile station 104 (i) another HDM providing a new active set for mobilestation 104 and directing the mobile station to hand off to the newactive set member (i.e., the sector determined to have a signal strengththat exceeds T_ADD by T_COMP) and (ii) a new NLUM that may be includedin the HDM or may be its own separate message. The new NLUM may containa different plurality of neighbor lists corresponding to the mobilestation's revised active set.

To facilitate communication between served mobile stations and remoteentities 114 on the PTSN, each BSC 108 may be coupled with an MSC orother switch 110 that provides connectivity with the PSTN 112. Each BSC108 may also be coupled with a PDSN or other gateway 116 that providesconnectivity with a packet-switched network 118, so that served mobilestations can communicate with remote entities 120 on the packet-switchednetwork.

The various components of RAN 102 (e.g., BTS 106 and BSC 108) maycomprise any number of functional elements. FIG. 2 is a simplified blockdiagram showing some of the functional elements 128 that can be includedin one or more (or a combination) of the components of RAN 102, inaccordance with an exemplary embodiment. As shown, the functionalelements 128 include a wireless network interface 130, a processor 132,and data storage 134, all of which may be communicatively linkedtogether by a system bus, network, or other connection mechanism 136.

Wireless network interface 130 functions to facilitate air interfacecommunication with mobile stations served by RAN 102. Such air interfacecommunication may be carried out according to one or more protocols suchas those noted above. RAN 102 may send various messages to served mobilestations via wireless network interface 130. For instance, RAN 102 maysend to mobile station 104 via network interface 130, theabove-discussed NLUMs and HDMs. Further, RAN 102 may receive variousmessages from served mobile stations via network interface 130, such asthe above-discussed PSMMs and HCMs.

Processor 132 comprises one or more general purpose processors, such asINTEL processors or the like, and/or one or more special purposeprocessors, such as digital signal processors or application specificintegrated circuits. To the extent processor 132 includes more than oneprocessor, the processors could work separately or in combination. Datastorage 134, in turn, comprises one or more volatile or non-volatilestorage components, such as optical, magnetic, or organic storage, anddata storage 134 can be integrated in whole or in part with processor132.

As shown, data storage 134 comprises program logic 138 and neighbor listdata 140. Program logic 138 may comprise machine language instructionsor other logic executable or interpretable by processor 132 to carry outvarious RAN functions described herein. Although depicted separate fromwireless network interface 130, program logic 138 may be provided asfirmware or other logic integrated directly within wireless networkinterface 130.

Neighbor list data 140 may include data representing multiple differentneighbor lists each optimized for different relative signal strength ofsectors provided by RAN 102 such that each neighbor list corresponds toa respective rank-ordering of the respective active set members ofmobile stations served by RAN 102. For instance, given sectors A, B, andC (i.e., the active set members of mobile station 104), neighbor listdata 140 may include, among other data, data representing one neighborlist that is optimized for a relative signal strength in which sector Ahas the strongest signal strength, sector B has the second strongestsignal strength, and sector C has the weakest signal strength.Accordingly, the neighbor list would correspond to the rank-orderingA-B-C. Further, neighbor list data 140 may include, among other data,data representing another neighbor list that is optimized for adifferent relative signal strength in which sector A has the strongestsignal strength, sector C has the second strongest signal strength, andsector B has the weakest signal strength. Accordingly, this neighborlist would correspond to the rank-ordering A-C-B.

Table 1 below illustrates one example of neighbor list data 140 suitablefor system 100. As shown in Table 1, exemplary neighbor list data 140includes six different neighbor lists (i.e., Neighbor Lists 1-6). Eachof the neighbor lists may include (e.g., list) the same neighbors.Alternatively, the neighbors of one of the neighbor lists may differ byat least one neighbor from the neighbors of another one of the neighborlists. Referring to Table 1, Neighbor List 1 is optimized for a relativesignal strength in which sector A has the strongest signal strength,sector B has the second strongest signal strength, and sector C has theweakest signal strength, Neighbor List 2 is optimized for a relativesignal strength in which sector A has the strongest signal strength,sector C has the second strongest signal strength, and sector B has theweakest signal strength, Neighbor List 3 is optimized for a relativesignal strength in which sector B has the strongest signal strength,sector A has the second strongest signal strength, and sector C has theweakest signal strength, and so forth.

TABLE 1 Neighbor List Data 140. Relative Signal Strength of the MobileStation's Active Set Members (i.e., Rank-Ordering) Neighbor Strongest2^(nd) Strongest Weakest List Signal Strength Signal Strength SignalStrength 1 A B C 2 A C B 3 B A C 4 B C A 5 C A B 6 C B A

In addition to corresponding with a different respective rank-orderingof the mobile station's active set members, each of the multipledifferent neighbor lists included in neighbor list data 140, such asNeighbor Lists 1-6 shown in Table 1, may define a different respectivepriority scanning order of various sectors neighboring the mobilestation's active set members, such as neighbors N shown in FIG. 1.Defining a priority scanning order of various neighbor sectors maycomprise listing the various neighbors in a particular order.

As noted above, the priority scanning order of a neighbor list may bebased on the relative likelihood that a mobile station will hand off tothe various neighbors listed in the neighbor list. In one example, forinstance, given mobile station 104 having active set members A, B, and Cwith a rank-ordering of A-B-C, the likelihood that the mobile stationwill hand off to one of A's neighbors may be greater than the likelihoodthat the mobile station will hand off to one of B's neighbors, which maybe greater than the likelihood that the mobile station will hand off toone of C's neighbors. Therefore, the priority scanning order defined bya neighbor list that corresponds to the rank-ordering A-B-C, such asNeighbor List 1 shown in Table 1, may list A's neighbors first, followedby B's neighbors, and then C's neighbors. Likewise, the priorityscanning order defined by a neighbor list that corresponds to therank-ordering A-C-B, such as Neighbor List 2 shown in Table 1, may listA's neighbors first, followed by C's neighbors, and then B's neighbors.

Also as noted above, each of the multiple different neighbor listsincluded in neighbor list data 140, such as Neighbor Lists 1-6 shown inTable 1, may include any number of respective neighbors per active setmember. For instance, each neighbor list may include an equal number ofneighbors per active set member. Alternatively, each neighbor list mayinclude a different number of neighbors per active set member, perhapsbased on the relative signal strength of the mobile station's active setmembers. For example, a neighbor list, such as Neighbor List 1 shown inTable 1, that is optimized for a relative signal strength in whichsector A has the strongest signal strength, sector B has the secondstrongest signal strength, and sector C has the weakest signal strength,may include a greater number of A's neighbors than B's neighbors, and agreater number of B's neighbors than C's neighbors. Accordingly,Neighbor List 1 may include five of A's neighbors, four of B'sneighbors, and three of C's neighbors.

Moreover, the difference between the respective number of neighbors peractive set member of one of the neighbor lists may be different than thedifference between the respective number of neighbors per active setmember of another one of the neighbor lists. For instance, thedifference between the respective number of neighbors per active setmember of Neighbor List 1 may be equal to one as in the above example,however, the difference between the respective number of neighbors peractive set member of Neighbor List 2 may be equal to two. For example,Neighbor List 2 may include six of A's neighbors, four of C's neighbors,and two of B's neighbors.

FIG. 3 is a simplified block diagram showing some of the functionalcomponents that can be included in a mobile station, such as mobilestation 104, in accordance with an exemplary embodiment. As shown,mobile station 104 includes a wireless communication interface 150, auser interface 152, a processor 154, and data storage 156, all of whichmay be communicatively linked together by a system bus, network, orother connection mechanism 158.

Wireless communication interface 150 functions to facilitate airinterface communication with RAN 102. For example, mobile station 104may receive various messages from RAN 102 via wireless communicationinterface 150, such as the above-discussed HDMs and NLUMs. Further,mobile station 104 may send various messages to RAN 102 via wirelesscommunication interface 150, such as the above-discussed PSMMs and HCMs.Such air interface communication may be carried out according to one ormore protocols such as those noted above. For instance, in oneembodiment, wireless communication interface 150 may support CDMAcommunication. As such, wireless communication interface 150 may includea transceiver having a rake receiver (not shown) of the type well knownin the art, which enables mobile station 104 to tune to particular PNoffsets, so as to facilitate scanning various sectors, and so forth. Anexemplary wireless communication interface is an MSM series chipsetmanufactured by Qualcomm Incorporated, together with one or moreinternal or external antennas.

User interface 152 includes components for receiving input from a userof mobile station 104 and providing output to a user of mobile station104. For instance, user interface 152 may include a keypad,touch-sensitive screen, microphone, and camera for receiving user input,and a display screen and speaker for providing user output. Further,user interface 152 may include analog/digital conversion circuitry tofacilitate conversion between analog user input/output and digitalsignals on which mobile station 104 can operate.

Processor 154 comprises one or more general purpose processors, such asINTEL processors or the like, and/or one or more special purposeprocessors, such as digital signal processors or application specificintegrated circuits. To the extent processor 154 includes more than oneprocessor, the processors could work separately or in combination. Datastorage 156, in turn, comprises one or more volatile or non-volatilestorage components, such as optical, magnetic, or organic storage, anddata storage 156 can be integrated in whole or in part with processor154.

As shown, data storage 156 includes reference data 160 and program logic162. Reference data 160 may include, among other data, active set data164, rank-ordering data 166, and neighbor list data 168. Active set data164, in turn, comprises data representing the mobile station's currentactive set members (i.e., those neighbors in which the mobile station iscurrently set to communicate), such as active set members A, B, and C.As noted above, mobile station 104 may receive a new set of active setmembers each time the mobile station receives an HDM from RAN 102.Accordingly, the active set data 164 may be updated each time the mobilestation receives a new active set.

Rank-ordering data 166, in turn, comprises data representing the currentrank-ordering of the mobile station's current active set members. Asnoted above, the rank-ordering may vary from time to time as therelative signal strength of the mobile station's active set memberschanges, perhaps due to the mobile station changing location.

Neighbor list data 168, in turn, comprises data representing a pluralityof different neighbor lists, preferably received by mobile station 104in a single NLUM. As noted above, each of the neighbor lists is (i)optimized for a respective rank-ordering of the mobile station's activeset members and (ii) defines a different priority scanning order ofvarious neighbors. Additionally, as noted above, each neighbor list mayinclude the same neighbors or alternatively, the neighbors of one of theneighbor lists may differ from the neighbors of another one of theneighbor lists by at least one neighbor.

Further, as noted above, each of the neighbor lists may include anyrespective number of neighbors per active set member. In one example,each neighbor list may include an equal number of neighbors per activeset member. Alternatively, each neighbor list may include a differentrespective number of neighbors per active set member based on therelative signal strength of the mobile station's active set members.Moreover, the difference between the respective number of neighbors peractive set member of one neighbor list may be the same or different thanthe respective number of neighbors per active set member of anotherneighbor list. Table 1 and its accompanying description above illustrateone example of a plurality of neighbors lists that may be represented byneighbor list data 168.

Program logic 162, in turn, may comprise machine language instructionsor other logic executable or interpretable by processor 154 to carry outvarious functions described herein. As shown, for instance, programlogic 162 includes sector scanning logic 170, signaling logic 172,rank-ordering logic 174, and selecting logic 176. Although depictedseparate from wireless communication interface 150, program logic 162may be provided as firmware or other logic integrated directly withinwireless communication interface 150. Other arrangements are possible aswell.

Sector scanning logic 170 functions to cause mobile station 104 toregularly scan and measure (i.e., to regularly monitor) the signalstrength of various sectors, in search of a sector in which mobilestation 104 can operate (i.e., a sector with sufficient signalstrength). Such scanning includes scanning (i) the mobile station'scurrent active set members and (ii) the neighbors listed in a neighborlist specified by neighbor list data 168, where the neighbor list isoptimized for the current rank-ordering of the mobile station's activeset members. Sector scanning logic 170 may cause mobile station 104 toscan the various sectors in accordance with the process noted above. Assuch, mobile station 104 may (i) scan all of its active set members andthen scan a first (highest priority) neighbor sector, (ii) scan all ofits active sectors again and then scan a next (next priority) neighborsector, and so forth until the mobile station has scanned all of theneighbors included in the neighbor list optimized for the currentrank-ordering of the mobile station's active set members.

Signaling logic 172 functions to engage in control signaling with RAN102, such as with BSC 108 via wireless communication interface 150. Thissignaling may include the above-discussed PSMM, HDM, NLUM, and HCMsignaling for instance, among a variety of other signaling. Thus, forinstance, when mobile station 104 scans a neighbor sector having asufficiently strong pilot signal (e.g., a signal strength that exceedsT_ADD by T_COMP), processor 154 may execute signaling logic 172 to causemobile station 104 to send a PSMM message to BSC 108.

Rank-ordering logic 174 functions to rank-order the signal strengths ofthe mobile station's active set members. Mobile station 104 mayrank-order the signal strengths of the mobile station's active setmembers each time the mobile station measures their signal strength orperhaps periodically according to a predefined schedule. Given activeset members A, B, and C, if at time t_(o) the measured signal strengthof A is the strongest, B is the second strongest signal, and C is theweakest, then processor 154 may execute rank-ordering logic 172 todetermine that the rank-ordering of the active set members at time t_(o)is A-B-C. However, as noted above, the relative signal strength of themobile station's active set members may vary from time to time and thusthe rank-ordering may vary from time to time. For instance, at time t₁after time t_(o), the measured signal strength of A may be thestrongest, C the second strongest signal, and B the weakest.Accordingly, processor 154 may execute rank-ordering logic 174 todetermine that the rank-ordering of the active set members at time t₁ isA-C-B. If the mobile station detects that the rank-ordering of themobile station's active set members has changed from the rank-orderingspecified in rank-ordering data 166, then rank-ordering logic 174 mayupdate rank-ordering data 166.

Each time mobile station 104 determines a rank-ordering of its activeset members, and each time it detects a change in the rank-orderingthereafter, selecting logic 176 functions to select, based on thedetermined current rank-ordering of the mobile station's active setmembers, a neighbor list from the plurality of different neighbors listsspecified by neighbor list data 168. As noted above, each of thedifferent neighbor lists received by mobile station 104 is optimized fora different rank-ordering. For instance, with reference to Table 1,Neighbor List 1 is optimized for the rank-ordering A-B-C and NeighborList 2 is optimized for the rank-ordering A-C-B. Thus, if mobile station104 determines that the rank-ordering of its active set members isA-B-C, then selecting logic 176 will select Neighbor List 1. If at sometime after selecting Neighbor List 1, mobile station 104 detects achange in rank-ordering from A-B-C to, perhaps, A-C-B, then selectinglogic 176 will allow mobile station 104 to transition to Neighbor List 2by selecting that neighbor list. Once mobile station 104 has selected aneighbor list, sector scanning logic 170 would then proceed scanningvarious sectors using the selected neighbor list such that the neighborslisted in the selected neighbor list are scanned according to thepriority scanning order defined by the selected neighbor list.

2. Exemplary Operation

FIG. 4 is a flow chart depicting functions that can be carried out by anexemplary mobile station, such as mobile station 104, in accordance withan exemplary method. The method is preferably carried out in a mobilestation operative to maintain in data storage an active set thatspecifies a plurality of wireless coverage areas in which the mobilestation is currently set to communicate (i.e., the mobile station'sactive set members). As shown at block 402, the method involves themobile station regularly monitoring signal strength of each of themobile station's active set members and determining a rank-ordering ofthe active set members based on their relative signal strength. At block404, the method then involves the mobile station wirelessly receiving anNLUM that contains a plurality of different neighbor lists eachcorresponding with a respective rank-ordering of the mobile station'sactive set members. At block 406, the method involves the mobile stationselecting one of the neighbor lists based on a determined firstrank-ordering of the mobile station's active set members. And at block408, the method involves the mobile station scanning various neighborslisted in the selected neighbor list in accordance with the selectedneighbor list.

In another example, the method may further involve the mobile stationdetecting a change in rank-ordering from the determined firstrank-ordering (e.g., A-B-C) to another rank-ordering (e.g., A-C-B). Asnoted above, once a change in rank-ordering is detected, the mobilestation may select and transition to another one of the neighbor listsbased on the new rank-ordering and the mobile station may scan variousneighbors listed in the other selected neighbor list in accordance withthe other selected neighbor list. This process of detecting a change inrank-ordering, selecting another neighbor list, and scanning theneighbors listed in the other selected neighbor list in accordance withthe other selected neighbor list may be repeated any number of times.

In yet another example, the method may further involve the mobilestation carrying out the method while the mobile station is activelyengaged in a call (e.g., a voice call or data communication session) viathe RAN. The method would be particularly advantageous in such asituation, as it may help to reduce the likelihood that the mobilestation will experience a dropped call.

FIG. 5 is a flow chart depicting functions that can be carried out by anexemplary RAN, such as RAN 102, in accordance with an exemplary method.The method is preferably carried out in a RAN serving a mobile station,wherein the mobile station maintains in data storage an active setspecifying a plurality of wireless coverage areas in which the mobilestation is currently set to communicate (i.e., the mobile station'sactive set members), and wherein the mobile station regularly monitorssignal strength of each of the active set members and determines arank-ordering of the active set members based on their relative signalstrength.

As shown at block 502, the method involves the RAN detecting a triggerfor updating neighbor-list data in the mobile station. The triggerdetected by the RAN may include among other events, the RAN receiving arequest for handoff from the mobile station and/or the RAN receiving aPSMM from the mobile station. As noted above, such a PSMM may indicatethe respective signal strengths of the mobile station's active setmembers and the signal strength of at least one wireless coverage area,perhaps a neighbor sector, having a signal strength greater than atleast one of the respective signal strengths of the active set members.

At block 504, in response to detecting the trigger, the method involvesthe RAN sending to the mobile station an NLUM containing a plurality ofdifferent neighbor lists, wherein each neighbor list corresponds to adifferent respective rank-ordering of the mobile station's active setmembers and defines a different respective priority scanning order ofneighbors listed in the neighbor list.

In another example, the method may further involve the RAN receiving aPSMM from the mobile station and the RAN responsively sending an HDM tothe mobile station granting handoff, wherein the NLUM is sent to themobile station in the HDM.

3. Conclusion

Various exemplary embodiments have been described above. Those skilledin the art will understand, however, that changes and modifications maybe made to those examples, and that those examples may be combined,without departing from the scope of the claims.

What is claimed is:
 1. In a mobile station operative to maintain in datastorage an active set specifying a plurality of wireless coverage areas(“active set members”) in which the mobile station is currently set tocommunicate, a method comprising: the mobile station regularlymonitoring signal strength of each of the mobile station's active setmembers and determining a rank-ordering of the active set members basedon their relative signal strength, wherein relative signal strength ofthe active set members varies from time to time and thus therank-ordering varies from time to time; the mobile station wirelesslyreceiving a neighbor list update message (NLUM) that contains aplurality of different neighbor lists each listing respective neighborsand corresponding with a respective rank-ordering of the mobilestation's active set members; based on a determined first rank-orderingof the mobile station's active set members, the mobile station selectingone of the neighbor lists; the mobile station scanning the neighborslisted in the selected neighbor list in accordance with the selectedneighbor list; the mobile station detecting a change in therank-ordering from the determined first rank-ordering to anotherrank-ordering; based on the other rank-ordering, the mobile stationselecting another one of the neighbor lists; and the mobile stationscanning the neighbors listed in the selected other neighbor list inaccordance with the selected other neighbor list.
 2. The method of claim1, wherein each neighbor list defines a different priority scanningorder of the neighbors listed in the neighbor list.
 3. The method ofclaim 2, wherein the mobile station scanning the neighbors listed in theselected neighbor list in accordance with the selected neighbor listcomprises the mobile station scanning the neighbors listed in theselected neighbor list in accordance with the priority scanning order ofthe selected neighbor list.
 4. The method of claim 1, wherein each ofthe neighbors comprises a wireless coverage area neighboring one of themobile station's active set members, and wherein the neighbors of one ofthe neighbor lists differ by at least one neighbor from the neighbors ofanother one of the neighbor lists.
 5. The method of claim 1, whereineach of the neighbors comprises a wireless coverage area neighboring oneof the mobile station's active set members, and wherein all of theneighbor lists list the same neighbors but each neighbor list lists theneighbors in a different respective priority scanning order.
 6. Themethod of claim 1, wherein each neighbor list lists a respective numberof neighbors per active set member such that the respective number ofneighbors for each active set member is different.
 7. The method ofclaim 6, wherein each neighbor list defines a different differencebetween the respective numbers of neighbors per active set member. 8.The method of claim 1, wherein wirelessly receiving the NLUM compriseswirelessly receiving the NLUM from a radio access network serving themobile station.
 9. The method of claim 1, wherein each of the neighborsis a Code Division Multiple Access (CDMA) sector operating on arespective pseudo-noise (PN) offset, and wherein each neighbor listidentifies neighbors at least in part by PN offset.
 10. The method ofclaim 1, wherein the signal strength is a signal-to-noise ratio.
 11. Themethod of claim 1, further comprising carrying out the method while themobile station is actively engaged in a call.
 12. A mobile stationcomprising: a wireless communication interface for communicatingwirelessly with a radio access network (RAN), including wirelesslyreceiving a neighbor list update message containing a plurality ofdifferent neighbor lists each defining a priority scanning order ofneighbors listed in the neighbor list; a processor; data storage; activeset data stored in the data storage defining a plurality of wirelesscoverage areas (“active set members”) in which the mobile station iscurrently set to communicate; and program logic stored in the datastorage and executable by the processor for (i) monitoring signalstrength of each of the active set members (ii) determining a firstrank-ordering of the active set members based on their relative signalstrength, (iii) selecting one of the neighbor lists based on thedetermined first rank-ordering, (iv) scanning the neighbors listed inthe selected neighbor list for handoff, wherein the scanning is carriedout according to the priority scanning order, and (v) selecting anotherone of the neighbor lists in response to the mobile station detecting achange in the rank-ordering from the determined first rank-ordering toanother rank-ordering.
 13. The mobile station of claim 12, wherein eachof the neighbors is a Code Division Multiple Access (CDMA) sectoroperating on a respective pseudo-noise (PN) offset, and wherein eachneighbor list identifies neighbors at least in part by PN offset. 14.The mobile station of claim 12, wherein each neighbor list defines adifferent priority scanning order of the neighbors listed in theneighbor list.
 15. The mobile station of claim 12, wherein each of theneighbors comprises a wireless coverage area neighboring one of themobile station's active set members, and wherein the neighbors of one ofthe neighbor lists differ by at least one neighbor from the neighbors ofanother one of the neighbor lists.
 16. The mobile station of claim 12,wherein each of the neighbors comprises a wireless coverage areaneighboring one of the mobile station's active set members, and whereinall of the neighbor lists list the same neighbors but each neighbor listlists the neighbors in a different respective priority scanning order.17. The mobile station of claim 12, wherein each neighbor list lists arespective number of neighbors per active set member such that therespective number of neighbors for each active set member is different.18. The mobile station of claim 17, wherein each neighbor list defines adifferent difference between the respective numbers of neighbors peractive set member.
 19. The mobile station of claim 12, wherein thesignal strength is a signal-to-noise ratio.